1. Field of the Invention
The present invention relates to a method for stably preparing extremely high-purity aqueous hydrogen peroxide by safely purifying aqueous hydrogen peroxide containing inorganic impurities. Aqueous hydrogen peroxide purified by the present invention can be used particularly in a semiconductor manufacturing field.
2. Description of the Related Arts
At present, hydrogen peroxide has mainly been manufactured by an anthraquinone process, but in aqueous hydrogen peroxide prepared by this method, various kinds of inorganic impurities are contained. In general, several hundreds .mu.g/l of the inorganic impurities are contained in aqueous hydrogen peroxide having a concentration of 5 to 70% by weight which is a substantially practical concentration. On the other hand, in a semiconductor manufacturing field, the high-purity aqueous hydrogen peroxide is required, and in recent years, such extremely high-purity aqueous hydrogen peroxide that the concentration of the remaining inorganic impurities is about 0.05 .mu.g/l or less has been required.
Heretofore, as a technique for removing the inorganic impurities from aqueous hydrogen peroxide to purify the same, there is known a method which comprises bringing aqueous hydrogen peroxide into contact with an ion exchange resin. Most of metals contained in aqueous hydrogen peroxide can be removed therefrom by bringing aqueous hydrogen peroxide into contact with a strongly acidic cation exchange resin. Furthermore, chloride ion, sulfate ion and a part of metallic compounds can be removed by bringing aqueous hydrogen peroxide into contact with a strongly basic anion exchange resin.
That is to say, the purification of aqueous hydrogen peroxide can be carried out by using the strongly acidic cation exchange resin singly, the strongly basic anion exchange resin singly, a mixture thereof singly, or a combination of these ion exchange resins. Examples in which the purification of aqueous hydrogen peroxide can be carried out by the use of these ion exchange resins have been disclosed in Japanese Patent Publication No. 16677/1960, German Patent Application Laid-open Nos. 4214075 and 4222109, French Patent No. 2677011, and Japanese Patent Application Laid-open Nos. 17105/1993 and 172805/1995.
Particularly, in order to obtain, by the purification, such extremely high-purity aqueous hydrogen peroxide as to be required in the semiconductor manufacturing field, it is usual to use the cation exchange resin and the anion exchange resin together. That is to say, the anion exchange resin which can be used in the purification of aqueous hydrogen peroxide occupies an influential position together with the cation exchange resin.
In Japanese Patent Publication No. 16677/1960 mentioned above has described a method for purifying aqueous hydrogen peroxide through an anion exchange resin converted into a bicarbonate form by treating it with an aqueous sodium bicarbonate solution. However, when this method is merely used together with the cation exchange resin, there cannot be obtained the high-purity aqueous hydrogen peroxide having a metal impurity concentration of about 0.05 ppb by weight or less which has been required of late. That is to say, metals such as calcium, copper and iron cannot sufficiently be removed by this purification, so that they inconveniently remain in aqueous hydrogen peroxide.
Furthermore, in Japanese Patent Application Laid-open No. 172805/1995 mentioned above, there has been disclosed a method for purifying aqueous hydrogen peroxide through an anion exchange resin converted into a carbonate form or a bicarbonate form by treating the anion exchange resin with an aqueous ammonium carbonate or ammonium bicarbonate solution in order to prevent sodium from dissolving out from the anion exchange resin. Even by the use of this method, it is difficult to obtain the high-purity aqueous hydrogen peroxide having a metal impurity concentration of about 0.05 ppb by weight or less.